This invention relates to an ultrasonic transmitter and receiver, particularly to ultrasonic transmitters and receivers adapted for use in underwater voice communication.
The high ambient noise rejection capabilities of phase or frequency modulation systems, hereinafter generically referred to as angle modulation systems, give the system to be described advantages over existing amplitude modulation systems now employed in the field of underwater communications. Primarily, to produce a signal at the output stage of the receiver of equivalent signal-to-noise ratio to a comparable amplitude modulation system requires either less transmitted power at the same range or allows for increased range at the same transmitted power. This is an inherent property of angle modulation systems and results from the fact that the carrier amplitude is maintained substantially constant.
A further advantage arises due to the "capture effect" associated with wide band angle modulation systems. With a number of divers attempting to communicate simultaneously with identical units according to the invention, a situation which would result in intolerable cross-talk in amplitude modulation systems, any one diver will essentially hear only the diver nearest to him with little or no interference from the others. A relatively distant diver, or base station, however, may transmit and be received by any number of divers when the other transmitters are not in use. In this way, selective communication is facilitated, i.e. a large number of divers may communicate amongst themselves in pairs or in groups, and to and from surface or submerged vessels, at the same carrier center frequency. This feature largely eliminates the need for variable tuning.